The prevalence and genotype distributionofActinobacillus actinomycetemcomitansstrains in families where at least one adult family member (proband) suffered from periodontal disease was investigated to better understand how this periodontal organism is acquired or transmitted. Fifteen probands with severe (established) periodontal disease (EPD) and their 46 immediate family members were sampled forA. actinomycetemcomitans. Among the 15 families, 10 contained at least one additional family member colonized with oralA. actinomycetemcomitans. Genomic DNA from 3 subgingivalA. actinomycetemcomitansstrains from each of the 10 probands and their 17 family members were amplified and characterized by the polymerase chain reaction (PCR) using a single arbitrary primer known to distinguishA. actinomycetemcomitansstrains. The PCR products from each strain were separated by electrophoresis on a 1% submarine agarose gel containing ethidium bromide and visualized by UV light transillumination. The amplification products migrated to form readily distinguishable bands and, since the banding patterns were characteristic of strains ofA. actinomycetemcomitans, these patterns were called “amplitypes.” The culture studies showed that 51% of all patients suffering from EPD carried oralA. actinomycetemcomitans. Moreover, 50% of their spouses and 30% of their children harbored the bacterium. Comparison of the PCR‐generated amplitypes showed that 26 out of 27 individuals had strains exhibiting a single amplitype ofA. actinomycetemcomitans, the 27th being colonized by 2 different amplitypes. They also showed that in 6 out of 7 families, the husband and wife did not harbor the sameA. actinomycetemcomitansamplitype. Furthermore, most often children carried an amplitype identical to one of the parents. However, one of the 10 children harbored two amplitypes each of which was identical to the parents. We propose thatA. actinomycetemcomitansis acquired through contact with one of the parents at some time during childhood.J Periodontol 1994;65:2–7.